1. Field of the Invention
The present invention relates generally to a fluorescent lamp, and more particularly to a low wattage fluorescent lamp adapted to function with high frequency electronic ballasts already present in the marketplace.
2. Description of Related Art
T8 fluorescent lamps have become quite popular in North American markets, and have largely supplanted the previous generation T12 fluorescent lamps due to their inherent higher efficiency. A typical North American 4-foot T8 fluorescent lamp using the known three component rare earth phosphor blends operates on the IES reference circuit at 32.5 watts (W) and produces 2850 lumens or about 88 lumens/watt. On high frequency commercial electronic ballasts, efficiencies are significantly higher, near 100 lumens/watt.
It is desirable to improve the energy efficiency of T8 fluorescent lamps to consume less energy. There currently exist no low-wattage lamps that deliver the same lumen output as standard lamps. Because lighting applications employing T8 lamps account for a significant portion of total energy consumption in North America, an improved energy efficient lamp will significantly reduce total energy consumption. Reduced energy consumption translates into cost savings to the consumer as well as reduced environmental impact associated with excess energy production necessary to meet current needs.
One way to reduce energy costs for this lamp would be to replace current installed electronic ballasts with ballasts which operate the lamp at a lower current. However, simply lowering the lamp current will reduce light output and in certain lighting applications light levels cannot or are not desired to be reduced. A major problem associated with producing such an energy efficient system is that current lighting installations employ relatively expensive high-frequency electronic ballasts having long lives. Consequently, a low-wattage lamp must either be compatible with existing electronic ballasts, or require the replacement of such ballasts at consumer expense. Replacing the electronic ballasts would offset the energy cost savings, and therefore would be a disincentive for consumers.
Consequently, there is a need for a low-wattage T8 fluorescent lamp having equivalent lumen output compared with standard T8 fluorescent lamps, that is adapted to function with currently emplaced high-frequency electronic ballasts.
A low pressure mercury vapor discharge lamp is provided having a light-transmissive glass envelope with an inner surface, means for providing a discharge, an ultraviolet reflecting barrier layer of alumina particles coated on or adjacent the inner surface of the glass envelope, a phosphor layer coated on the barrier layer, and a discharge-sustaining fill of mercury and inert gas sealed inside the envelope. The inert gas is a mixture of argon and krypton, with krypton being 10-40 volume percent of the mixture. The total pressure of the inert gas is 1-4 torr. The lamp has a lumen efficiency of at least 80 lumens/watt.